This invention relates generally to optical discs used for data storage, and more specifically to inhibiting unauthorized copying of data from one disc to another.
Compact Discs (CD) and Digital Versatile Discs (DVD) exist in a variety of types. For mass distribution of copyrighted material, such as music and video, the discs are typically stamped from a master disc. Some disc types are writable, and can be written only once. Other disc types can be rewritten many times.
One proposed method for copy protection of stamped video media is the Content Scramble System (CSS) developed by Matsushita Electric Industries. In CSS, the video data is selectively scrambled. Descrambing requires decryption keys that are stored on the disc. Licensed drives can retrieve the keys and use the keys to unscramble the data. Keys can be passed only between licensed devices containing special hardware, for example, between a licensed drive and a licensed decoder in a computer. For general background information on CSS and DVD copy protection, see, for example, J. A. Bloom et al., xe2x80x9cCopy Protection for DVD Video,xe2x80x9d Proceedings of the IEEE, v87 n7, (Jul. 1, 1999), pp 1267-1276, and Alan E. Bell, xe2x80x9cThe Dynamic Digital Disk,xe2x80x9d IEEE Spectrum, v36 n10, (October 1999), pp 28-35.
A byte-for-byte copy of an entire DVD disc, including the area containing the decryption keys, could be used to create a copy of a disc that is indistinguishable from the original. Consequently, various roadblocks have been proposed to prevent a recording drive from recording the decryption keys. For example, in one proposed copy protection method, for recordable media, the area where the decryption keys need to be recorded would be pre-embossed, or in the case of write-once media, pre-recorded. A resulting copy would contain all of the scrambled data, but not the decryption keys required to unscramble the data. However, the keys are stored in Control Data blocks in the lead-in area of the disc, and the Control Data blocks contain other essential data that needs to be variable. Preventing all writing into the Control Data blocks limits the usefulness of the disc for legitimate recording of un-protected data. In addition, rewriteable media with pre-stamped lead-in areas may be incompatible with existing DVD-ROM and DVD video players, and may also increase manufacturing costs.
There is a need for a writeable disc medium that physically prevents reading of arbitrary data, with a correctable error rate, written in reserved areas, such as areas reserved for decryption keys, but still allows reading and writing of arbitrary data in non-reserved areas, and allows a combination of some arbitrary data and some pre-determined data in the reserved areas. There is a further need for such a medium that is fully backward compatible with existing drives.
The spatial structure of a particular area of the disc is modified, so that only limited data can be read, with a correctable error rate, from the particular area. If arbitrary data, such as decryption keys (which effectively appear as pseudo-random data), are written into the particular area, the data in the particular area will be effectively unreadable due to an uncorrectable error rate. However, a combination of some limited arbitrary data and pre-determined data may be written into a block, and may be read with a correctable error rate, enabling an un-protected disc to be written and played. Thus, compatibility with existing drives is preserved, without representing a threat of unauthorized copying of stamped discs.
Spatial features (notches, bumps, etc.) are implemented such that they intentionally distort the analog Read Data signal for any marks or spaces adjacent to the spatial features. When a transition between a mark and a space is not near a spatial feature, the distortion does not affect the resulting binary Read Data signal. In contrast, if a transition between a mark and a space is near a spatial feature, the timing of the resulting binary Read Data signal is affected significantly (sufficient to cause a data error). For calibration, a mark and space sequence may be repeatedly written adjacent to spatial features, while adjusting the timing of the Write Data signal until no data errors occur. From the calibration procedure, it is known when a transition between a mark and a space must be initiated in the Write Data signal in order to place the mark or space at a known spatial location.
When recording data on a disc, data is scrambled (along with error detection codes) and formed into two-dimensional blocks, where error correction codes (ECC) are added for rows and columns of the two-dimensional blocks (called ECC blocks). Data in a specified area (specified rows and columns) of an ECC block may be pre-determined. Spatial features as described above may be formed onto the disc at manufacturing time, so that only the pre-determined data (in scrambled form), and corresponding pre-determined ECC codes, can be written and read with a correctable error rate in the areas reserved for pre-determined data. That is, only the pre-determined data will have marks and spaces properly aligned with (transitions between marks and spaces not near) the spatial features. Arbitrary data in the specified area will have errors due to proximity of transitions between marks and spaces to spatial features, and will likely overwhelm the error-correction capability for the ECC block. The remaining parts of the ECC block may be written with arbitrary data. Accordingly, when pre-determined data is written into the specified area, the remaining part of each Control Data ECC block can contain arbitrary data, and the entire block can be read with a correctable error rate. Conversely, writing user determined (pseudo-random) data in the specified area will cause the entire Control Data ECC block to be uncorrectable, and therefore unreadable. In particular, Control Data ECC blocks may have the area reserved for decryption codes to be at least partially pre-determined, while still permitting some variable data to be written in other areas of the block.